The colony now consists of some 6,000 mice, and of these approximately 800 are employed as breeders. These mice are all in good health. We have had very few problems with the colony except sporadically the mice have had pinworm infections. We are producing approximately 350 mice per week, and the production will probably reach 400 per week some time this winter. More than half of these animals are used in experimental projects. We now have about 54 established tumor lines, and in recent months we have implanted 20 additional primary tumors. Of these latter tumors, we expect at least half to take. This will increase our number of certain tumor types. Nearly all of the tumors have been frozen, although we are still transplanting a number of them in the animals continuously since they are being used in experiments. We have begun to keep frozen the tumors that we are not using routinely. This gives us some additional space in the tumor room. An experienced experimental pathologist will be following the pathology of the human tumors in the colony to determine whether there is any histological change during continuing passage. We are planning to place colony data on a computer so that we can eventually determine whether there is any kind of genetic drift. We have also started some backcrossing so that eventually we will know whether the colony is completely homogenous. The colony has been found to be most suitable for experimental studies on new chemotherapeutic drugs such as interferon and anthracycline analogs. The tumors have been excellent sources of material for a number of biochemical investigations. The effect of leukemic viruses on human tumors grown in athymic mice is now under investigation. Comparison of cell lines derived from the human tumors which have been growing in athymic mice has continued. Some of the variations which occur when the tumor cells are grown in vitro have been documented. We have isolated clones from a human astrocytoma cell line (T24). The clones showed various degrees of tumorigenicity, and recent studies indicated that the degree of malignancy is related to the disintegration of karyology. This phenomenon will be further studied. The functions of a human colon carcinoma cell line (T84) have been investigated. Preliminary studies demonstrated that a vasoactive intestinal peptide stimulates the ion transport of these cells, demonstrating that they retain some colon functions. The studies will be continued with other human carcinoma cell lines. We expect to continue to improve the quality of the colony by methods which we have outlined above.